President Bush is defending the cost of a plan to change the way many older Americans pay for health care now that his own budget office says the reforms will cost one third more than originally estimated.

The president's plan to reform Medicare overcame Republican opposition in Congress only when legislators were assured that its cost would not exceed $400 billion.

Mr. Bush has made those reforms a cornerstone of his re-election campaign, saying he has surmounted decades of political deadlock by delivering lower prescription drug prices for senior citizens.

But the White House says the price of that plan over the next 10 years will be more than $100 billion higher than November's Congressional estimate. Administration officials say the president's budget to be released Monday will show Medicare reform costing at least $530 billion.

Asked about that difference, Mr. Bush said he first saw the new numbers two weeks ago and asked two important questions.

"One, does the Medicare reform do what we want it to do still, which is to provide modern medicine for our seniors and to introduce competition which will eventually hold down costs of Medicare? And secondly, did the new estimate of Medicare costs fulfill my promise to reduce the deficit by half over a five-year period of time?" he asked.

The president says the plan will meet his deficit reduction goals if Congress is "wise with the taxpayers' money" and willing to make tough choice about what to fund.

White House spokesman Scott McClellan says the president did not know about the higher costs when he was negotiating the bill with Congress. He says the administration's Office of Management and Budget did not receive cost estimates from the Department of Health and Human Services until after the legislation was passed.